The present invention relates generally to a low inductance large area coil for an inductively coupled plasma source. More specifically, the present invention relates to a low inductance large area coil as a source for generating a plasma which can be used for treating semiconductor wafers in low pressure processing equipment.
Plasma generation is useful in a variety of semiconductor fabrication processes, for example enhanced etching, deposition, etc. Plasmas are generally produced from a low pressure gas by inducing an electron flow which ionizes individual gas molecules through the transfer of kinetic energy through individual electron-gas molecule collisions. The electrons are commonly accelerated in an electric field, typically a radio frequency (RF) electric field.
Numerous methods have been proposed to accelerate the electrons in the RF electric field. One method is to excite electrons between a pair of opposed electrodes which are oriented parallel to the wafer in a processing chamber. The use of an electric field normal to the wafer does not provide efficient conversion of the kinetic energy to ions, since a large portion of the electron energy is dissipated through electron collisions with the walls of the processing chamber or with the semiconductor wafer itself.
A more efficient technique for exciting electrons in the RF field is to use a single winding coil (SWC) parallel to the plane of the wafer and the plasma to excite the electrons. U.S. Pat. No. 4,948,458 discloses a device for employing such a technique, which is depicted in FIGS. 1-3. As shown in FIG. 1, a plasma generating device includes an enclosure 10 with an access port 12 formed in an upper wall 14. A dielectric shield 16 is disposed below the upper wall 14 and extends across the access port 12. The dielectric shield 16 is sealed to the wall 14 to define a vacuum-tight interior of the enclosure 10. A planar single winding coil (SWC) 20 is disposed within the access port 12 adjacent the dielectric shield 16 and oriented parallel to the wafer W which is supported by a surface 22. A process gas is introduced into the chamber 18 through a port 24 formed on side of the enclosure 10.
FIG. 2 depicts schematically the plasma generating device illustrated in FIG. 1. As shown in FIGS. 1 and 2, an RF source 30 is coupled via a coaxial cable 32 through an impedance matching circuit 35 to the SWC 20. The impedance matching circuit 35 includes a primary coil 36 and a secondary loop 38 which may be positioned to adjust the effective coupling of the circuit and allow for loading of the circuit at the frequency of operation, thereby maximizing power transfer. The primary coil 36 is mounted on a disk 40 which may be rotated about a vertical axis 42 in order to adjust the coupling. A tuning capacitor 44 is provided in series with the secondary loop 38 to adjust the circuit resonant frequency to the RF driving frequency. Another capacitor 34 is provided to cancel part of the inductive reactance of the primary coil 36 in the circuit. By resonating an RF current at a resonant frequency tuned to typically 13.56 Mhz through the coil 20, a planar magnetic field is induced, which penetrates the dielectric shield 16. The magnetic field causes a circulating flow of electrons between the coil 20 and the wafer W. The circulating flow of electrons makes it less likely that the electrons will strike the enclosure wall 10 between the coil 20 and the wafer W, and since the electrons are confined to a plane parallel to the planar coil 20, the transfer of kinetic energy in non-planar directions is minimized.
Shown in detail in FIG. 3, the SWC 20 comprises a singular conductive element formed into a planar spiral or a series of concentric rings. As shown in FIGS. 1 and 3, the SWC 20 also includes a center tap labeled (+) and an outer tap labeled (-), so that it can be connected to the circuitry of the plasma generating device.
In certain applications, such as the production of 400 mm wafers or large area flat panel displays, a large area plasma is needed. In order to produce a large area plasma, the area or diameter of the SWC 20 shown in FIGS. 1-3 must be increased. For a fixed pitch between turns, the SWC 20 increases in inductance if turns are added to increase the diameter. At large diameters, the SWC 20 produces a high inductance, which reduces the self-resonating frequency of the SWC 20. As the self-resonating frequency becomes nearer the radio frequency (RF) driving frequency, which is normally 13.56 MHz, impedance matching becomes more and more difficult. This is because it is hard to exactly match impedance within a small frequency range, due to the increased sensitivity of the match condition to changes in the settings of the impedance matching components. Therefore, a difficulty arises in maximizing power transfer when using an SWC to generate a larger area plasma.